diff --git a/wp-admin/ms-edit.php b/wp-admin/ms-edit.php
index 8cab8aaa7d..395bf71a31 100644
--- a/wp-admin/ms-edit.php
+++ b/wp-admin/ms-edit.php
@@ -23,7 +23,7 @@ switch ( $_GET['action'] ) {
 		if ( empty( $_POST ) )
 			wp_die( __("You probably need to go back to the <a href='ms-options.php'>options page</a>") );
 
-		if ( isset($_POST['WPLANG']) )
+		if ( isset($_POST['WPLANG']) && ( '' === $_POST['WPLANG'] || in_array($_POST['WPLANG'], get_available_languages()) ) )
 			update_site_option( "WPLANG", $_POST['WPLANG'] );
 
 		if ( is_email( $_POST['admin_email'] ) )
diff --git a/wp-admin/ms-options.php b/wp-admin/ms-options.php
index 2bc3efc356..df4c176054 100644
--- a/wp-admin/ms-options.php
+++ b/wp-admin/ms-options.php
@@ -252,19 +252,15 @@ if (isset($_GET['updated'])) {
 		<h3><?php _e('Site Wide Settings <em>(These settings may be overridden by blog owners)</em>') ?></h3>
 		<table class="form-table">
 			<?php
-			$lang_files = array();
-			if ( is_dir( ABSPATH . LANGDIR ) && $dh = opendir( ABSPATH . LANGDIR ) )
-				while( ( $lang_file = readdir( $dh ) ) !== false )
-					if ( substr( $lang_file, -3 ) == '.mo' )
-						$lang_files[] = $lang_file;
+			$languages = get_available_languages();
 			$lang = get_site_option('WPLANG');
-			if ( !empty($lang_files) ) {
+			if ( !empty($languages) ) {
 				?>
 				<tr valign="top">
 					<th width="33%"><?php _e('Default Language') ?></th>
 					<td>
 						<select name="WPLANG" id="WPLANG">
-							<?php mu_dropdown_languages( $lang_files, get_site_option('WPLANG') ); ?>
+							<?php mu_dropdown_languages( $languages, get_site_option('WPLANG') ); ?>
 						</select>
 					</td>
 				</tr>
diff --git a/wp-includes/l10n.php b/wp-includes/l10n.php
index efb9ad5ac6..a7c0a755a3 100644
--- a/wp-includes/l10n.php
+++ b/wp-includes/l10n.php
@@ -484,4 +484,30 @@ function &get_translations_for_domain( $domain ) {
 function translate_user_role( $name ) {
 	return translate_with_gettext_context( before_last_bar($name), 'User role' );
 }
+
+/**
+ * Get all available languages based on the presence of *.mo files in a given directory. The default directory is WP_LANG_DIR.
+ *
+ * @since 3.0.0
+ *
+ * @param string $dir A directory in which to search for language files. The default directory is WP_LANG_DIR.
+ * @return array Array of language codes or an empty array if no languages are present.  Language codes are formed by stripping the .mo extension from the language file names.
+ */
+function get_available_languages( $dir = null ) {
+	$languages = array();
+
+	if ( empty($dir) )
+		$dir = WP_LANG_DIR;
+
+	if ( is_dir( $dir ) && $dh = opendir( $dir ) ) {
+		while ( ( $lang_file = readdir( $dh ) ) !== false ) {
+			if ( substr( $lang_file, -3 ) == '.mo' )
+				$languages[] = basename($lang_file, '.mo');
+		}
+		closedir($dh);
+	}
+
+	return $languages;
+}
+
 ?>